CN115087704B

CN115087704B - Composition, binder, electrode mixture, electrode, and secondary battery

Info

Publication number: CN115087704B
Application number: CN202180013318.1A
Authority: CN
Inventors: 井口贵视; 北原隆宏; 矢野辽一; 浅野和哉; 新井佳奈子
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 2020-02-28
Filing date: 2021-03-01
Publication date: 2024-07-30
Anticipated expiration: 2041-03-01
Also published as: EP4112689A1; TWI820401B; JPWO2021172586A1; CN115087704A; JP2024026580A; TW202139507A; US20220407075A1; WO2021172586A1; KR20220132569A; EP4112689A4

Abstract

The present invention provides a composition comprising polyvinylidene fluoride (a) and a vinylidene fluoride polymer (B) (excluding polyvinylidene fluoride (a)), the polyvinylidene fluoride (a) comprising vinylidene fluoride units and formula (1): CH ₂＝CH-(CH)₂ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations) and the content of vinylidene fluoride units of polyvinylidene fluoride (A) is 95.0 to 99.99 mol% relative to the total monomer units of polyvinylidene fluoride (A), and the content of the above-mentioned pentenoic acid units of polyvinylidene fluoride (A) is 0.01 to 5.0 mol% relative to the total monomer units of polyvinylidene fluoride (A).

Description

Composition, binder, electrode mixture, electrode, and secondary battery

Technical Field

The present invention relates to a composition, a binder, an electrode mixture, an electrode, and a secondary battery.

Background

Patent document 1 proposes the use of a linear semi-crystalline copolymer [ polymer (a) ] comprising repeating units derived from a hydrophilic (meth) acrylic Monomer (MA) such as vinylidene fluoride (VDF) monomer and acrylic acid, which contains 0.05 to 10 mol% of repeating units derived from the hydrophilic (meth) acrylic Monomer (MA) and is characterized by a fraction of at least 40% of randomly distributed units (MA), as a binder for forming an electrode of a lithium battery and/or an electric double layer capacitor.

In patent document 2, a composition (C) is proposed, which contains:

-at least one semi-crystalline fluoropolymer [ polymer (F1) ] comprising recurring units derived from vinylidene fluoride (VDF) in an amount of at least 50% by mole relative to the total moles of recurring units of polymer (F1) and recurring units derived from at least one functional hydrogen-containing monomer comprising at least one hydrophilic (meth) acrylic Monomer (MA) having formula (I) in an amount of at least 0.1% by mole, preferably at least 0.3% by mole, even more preferably at least 0.5% by mole, and below 5% by mole relative to the total moles of recurring units of polymer (F1).

Formula (I):

[ chemical 1]

(Wherein:

r ₁、R₂ and R ₃, identical or different from each other, are independently selected from the group consisting of a hydrogen atom and a C ₁～C₃ hydrocarbon group,

-R _OH is a hydrogen atom or a C ₁～C₅ hydrocarbon moiety comprising at least one hydroxyl group

The above polymer (F1) has an intrinsic viscosity higher than 1.4dl/g, preferably higher than 2dl/g, and even more preferably higher than 2.5dl/g and lower than 5dl/g, measured in dimethylformamide at 25 ℃; and

At least one fluoropolymer [ polymer (F2) ] different from (F1), comprising recurring units derived from vinylidene fluoride (VDF) in an amount of at least 50% by mole relative to the total moles of recurring units of polymer (F2) and recurring units derived from at least one Fluorinated Monomer (FM) different from vinylidene fluoride in an amount of at least 2.5% by mole, preferably at least 4.0% by mole, and even more preferably at least 6% by mole relative to the total moles of recurring units of polymer (F2),

Wherein the polymer (F1) forms at least 10% by weight with respect to the total weight of the composition (C) and the polymer (F2) forms at most 90% by weight with respect to the total weight of the composition (C).

Patent document 3 proposes a binder composition for binding an electrode active material to a current collector coated with the electrode active material, the binder composition comprising a 1 st vinylidene fluoride polymer having an intrinsic viscosity of 1.7dl/g or more and a2 nd vinylidene fluoride polymer containing acrylic acid or methacrylic acid as a monomer unit.

Prior art literature

Patent literature

Patent document 1: international publication No. 2008/129041

Patent document 2: international publication No. 2018/073277

Patent document 3: international publication No. 2017/056974

Disclosure of Invention

Problems to be solved by the invention

The present invention aims to provide a composition which can form an electrode excellent in electrolyte swelling resistance and adhesion to a metal foil and excellent in flexibility, and further can provide an electrode mixture which is less likely to increase in viscosity.

Means for solving the problems

According to the present invention there is provided a composition comprising polyvinylidene fluoride (a) and a vinylidene fluoride polymer (B) (excluding polyvinylidene fluoride (a)), the polyvinylidene fluoride (a) comprising vinylidene fluoride units and formula (1): CH ₂＝CH-(CH)₂ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations) and the content of vinylidene fluoride units of polyvinylidene fluoride (A) is 95.0 to 99.99 mol% relative to the total monomer units of polyvinylidene fluoride (A), and the content of the above-mentioned pentenoic acid units of polyvinylidene fluoride (A) is 0.01 to 5.0 mol% relative to the total monomer units of polyvinylidene fluoride (A).

In the composition of the present invention, the vinylidene fluoride polymer (B) preferably contains a vinylidene fluoride unit and a unit based on a monomer copolymerizable with vinylidene fluoride excluding vinylidene fluoride and pentenoic acid represented by formula (1).

In the composition of the present invention, the vinylidene fluoride polymer (B) preferably contains vinylidene fluoride units, and fluorinated monomer units (excluding vinylidene fluoride units).

In the composition of the present invention, the above-mentioned fluorinated monomer unit of the vinylidene fluoride polymer (B) is preferably a unit based on at least one monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and hexafluoropropylene.

In the composition of the present invention, it is preferable that the content of the vinylidene fluoride unit of the vinylidene fluoride polymer (B) is 57.0 mol% to 99.9 mol% with respect to the total monomer units of the vinylidene fluoride polymer (B), and the content of the fluorinated monomer unit of the vinylidene fluoride polymer (B) is 0.1 mol% to 43.0 mol% with respect to the total monomer units of the vinylidene fluoride polymer (B).

In the composition of the present invention, the mass ratio ((A)/(B)) of the polyvinylidene fluoride (A) to the vinylidene fluoride polymer (B) is preferably 95/5 to 45/55.

In addition, according to the present invention, there is provided a binder containing the above composition.

Further, according to the present invention, there is provided an electrode mixture comprising the above composition or the above binder, a powder electrode material, and an aqueous or nonaqueous solvent.

Further, according to the present invention, there is provided an electrode comprising the above composition or the above binder.

Further, according to the present invention, there is provided a secondary battery including the above-described electrode.

Further, according to the present invention, there is provided a polyvinylidene fluoride (a) for producing a composition containing a polyvinylidene fluoride (a) and a vinylidene fluoride polymer (B) (wherein the polyvinylidene fluoride (a) is excluded), wherein the polyvinylidene fluoride (a) contains a vinylidene fluoride unit and formula (1): CH ₂＝CH-(CH)₂ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations) and the content of vinylidene fluoride units of polyvinylidene fluoride (A) is 95.0 to 99.99 mol% relative to the total monomer units of polyvinylidene fluoride (A), and the content of the above-mentioned pentenoic acid units of polyvinylidene fluoride (A) is 0.01 to 5.0 mol% relative to the total monomer units of polyvinylidene fluoride (A).

The polyvinylidene fluoride (A) of the present invention is preferably used for preparing a binder containing the above composition.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide a composition which can form an electrode excellent in electrolyte swelling resistance and adhesion to a metal foil and excellent in flexibility, and further can obtain an electrode mixture which is less likely to increase in viscosity.

Detailed Description

The following describes specific embodiments of the present invention in detail, but the present invention is not limited to the following embodiments.

The composition of the present invention contains polyvinylidene fluoride (PVdF) (a) and vinylidene fluoride polymer (VdF polymer) (B) (excluding PVdF (a)). In the composition of the present invention, as PVdF (a), a composition containing a vinylidene fluoride (VdF) unit and formula (1): CH ₂＝CH-(CH)₂ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations).

Conventionally, the following techniques have been known: as a binder for forming electrodes of secondary batteries, capacitors, and the like, a binder containing PVdF into which an acrylic acid unit or the like is introduced is used, thereby improving adhesion between the binder and a metal foil (current collector). However, there is a need for a composition (binder) that can form an electrode that is excellent in electrolyte swelling resistance and flexibility while ensuring sufficient adhesion to a metal foil (current collector), and that can provide an electrode mixture that is less prone to increase in viscosity.

It has been found that: the content of VdF units and the content of pentenoic acid units are further adjusted to be within specific ranges by selecting pentenoic acid units as monomer units to be introduced into PVdF (a), whereby sufficient adhesion between the PVdF (a) -containing composition and a metal foil (current collector) can be ensured and the electrolyte swelling resistance of the PVdF (a) -containing composition can be significantly improved. It was also found that: by using VdF polymer (B) together with PVdF (a), an electrode excellent in flexibility can be formed, and an electrode mixture with less tendency to increase in viscosity can be obtained. The composition of the present invention has been completed based on these technical ideas.

Further, since the content of pentenoic acid units in PVdF (a) is appropriately adjusted in the composition of the present invention, by using the composition of the present invention, the electrode material layer and the metal foil (current collector) are sufficiently adhered, and an electrode having sufficient holding power of the powder electrode material can be formed. Further, by using the composition of the present invention, although PVdF (a) and VdF polymer (B) are contained at high concentrations, a positive electrode mixture having an appropriate viscosity and less apt to rise even when the viscosity is stored for a long period of time can be produced, and thus an electrode exhibiting excellent characteristics can be produced with high productivity.

In addition, in a secondary battery using a conventional binder, when the secondary battery is stored at a high temperature, the polar group of the binder may be decomposed, and the resistance may be increased. The composition of the present invention contains PVdF (a) in which the content of pentenoic acid units is appropriately adjusted, and thus can suppress the influence of the decomposition of polar groups in a high-temperature environment. Therefore, by using the composition of the present invention, an electrode in which the electrode material layer and the metal foil (current collector) are sufficiently adhered can be formed, and a secondary battery in which it is difficult to increase the resistance value even when stored at high temperature can be produced.

The rate of increase in the resistance value of the secondary battery can be determined, for example, by the following method. The secondary battery (cell) placed in a constant temperature bath at 25℃was charged to 4.4V by a constant current-constant voltage method at a rate of 0.5C to 0.05C, and then the initial resistance value was measured by an AC impedance meter. Then, after the cells were stored in a constant temperature bath at 40 ℃ for 1 week, the cells were placed in a constant temperature bath at 25 ℃ for 3 hours, the cell temperature was lowered to 25 ℃, and the resistance value of the cells after the endurance test was measured. The average value of 5 cells was used as a measurement value, and the increase rate (%) of the resistance value after the endurance test relative to the initial resistance value (resistance value after the endurance test-initial resistance value)/initial resistance value×100 was obtained.

In addition, in a secondary battery using a conventional binder, when the secondary battery is repeatedly charged and discharged, the powder electrode material may peel off from the electrode material layer, and the discharge capacity may be reduced. The composition of the present invention contains PVdF (A) with the content of pentenoic acid units properly adjusted. Therefore, by using the composition of the present invention, even if charge and discharge are repeated, the powder electrode material is less likely to peel off from the electrode material layer, and a secondary battery (a secondary battery having a high capacity retention rate) capable of maintaining a sufficient discharge capacity can be produced.

The capacity retention rate of the secondary battery can be evaluated by the following method, for example. The secondary battery was held between plates and pressurized, and after constant-current-constant-voltage charging (hereinafter referred to as CC/CV charging) at 25 ℃ at a current corresponding to 0.5C (0.1C cut-off) to 4.2V, the secondary battery was discharged at a constant current of 0.5C to 3.0V, and the initial discharge capacity was obtained from the discharge capacity at the 3 rd cycle as 1 cycle. Here, 1C represents a current value for discharging the reference capacity of the battery for 1 hour, and for example, 0.5C represents a current value of 1/2 thereof. Under the same conditions as above, a cycle test of 300 cycles was performed at an operating voltage of 3.0 to 4.2V. The discharge capacity at the 300 rd cycle when the initial discharge capacity at the 3 rd cycle was 100% was used as the capacity maintenance rate.

Polyvinylidene fluoride (PVdF) (a) contains vinylidene fluoride (VdF) units and pentenoic acid units.

The pentenoic acid unit contained in PVdF (a) is based on formula (1): CH ₂＝CH-(CH)₂ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations).

In the formula (1), Y represents an inorganic cation and/or an organic cation. Examples of the inorganic cation include a cation such as H, li, na, K, mg, ca, al, fe. Examples of the organic cation include NH₄、NH₃R⁵、NH₂R⁵ ₂、NHR⁵ ₃、NR⁵ ₄(R⁵ independently represent an alkyl group having 1 to 4 carbon atoms) and the like. As Y, at least one selected from the group consisting of H, li, na, K, mg, ca, al and NH ₄ is preferable, at least one selected from the group consisting of H, li, na, K, mg, al and NH ₄ is more preferable, at least one selected from the group consisting of H, li, al and NH ₄ is further preferable, and H is particularly preferable. For convenience, specific examples of the inorganic cations and the organic cations are omitted from the description by reference numerals and valence.

The PVdF (a) contains 0.01 to 5.0 mol% of pentenoic acid units relative to the total monomer units of the PVdF (a). By properly adjusting the content of the pentenoic acid unit in PVdF (a), an electrode having more excellent electrolyte swelling resistance and adhesion to a metal foil and more excellent flexibility can be formed, and an electrode mixture having less tendency to increase in viscosity can be obtained.

The content of pentenoic acid units of PVdF (a) is 0.01 mol% to 5.0 mol%, preferably less than 5.0 mol%, more preferably 3.0 mol% or less, further preferably 2.0 mol% or less, particularly preferably 1.5 mol% or less, most preferably 1.0 mol% or less, and most preferably 0.05 mol% or more, based on the entire monomer units of PVdF (a). When the content of the pentenoic acid unit is within the above range, an electrode having more excellent electrolyte swelling resistance and adhesion to a metal foil and more excellent flexibility can be formed, and further an electrode mixture having less tendency to increase in viscosity can be obtained.

The VdF unit content of PVdF (a) is preferably 95.0 mol% to 99.99 mol%, more preferably more than 95.0 mol%, still more preferably 97.0 mol% or more, still more preferably 98.0 mol% or more, particularly preferably 98.5 mol% or more, most preferably 99.0 mol% or more, and still more preferably 99.95 mol% or less, based on the entire monomer units of PVdF (a). When the content of VdF unit is within the above range, an electrode having more excellent electrolyte swelling resistance and adhesion to a metal foil and more excellent flexibility can be formed, and an electrode mixture having less tendency to increase in viscosity can be obtained.

In the present invention, the PVdF composition can be measured by ¹⁹ F-NMR measurement, for example. The content of the pentenoic acid units of PVdF can be measured by using ¹ H-NMR measurement of PVdF after esterifying the carboxyl groups (-COOY) of the pentenoic acid units.

PVdF (a) may further contain a fluorinated monomer unit (VdF unit is not included therein). By further containing a fluorinated monomer unit, PVdF (a) can form an electrode having more excellent flexibility.

Examples of the fluorinated monomer include Tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), perfluoroalkyl ethylene, 2, 3-tetrafluoropropene, and trans-1, 3-tetrafluoropropene.

The fluorinated monomer is preferably at least one selected from the group consisting of CTFE, HFP, fluoroalkyl vinyl ether and 2, 3-tetrafluoropropene, more preferably at least one selected from the group consisting of CTFE, HFP and fluoroalkyl vinyl ether, and even more preferably at least one selected from the group consisting of HFP and fluoroalkyl vinyl ether, from the viewpoint of being capable of forming an electrode having more excellent flexibility.

The fluorovinyl ether is preferably a fluoroalkyl vinyl ether having a fluoroalkyl group having 1 to 5 carbon atoms, and more preferably at least one selected from the group consisting of perfluoro (methyl vinyl ether), perfluoro (ethyl vinyl ether) and perfluoro (propyl vinyl ether).

The PVdF (a) preferably contains no TFE unit, from the viewpoint of further improving the electrolyte swelling resistance and adhesion to the metal foil.

The content of the fluorinated monomer unit of PVdF (a) is preferably 0 to 4.99 mol%, more preferably 0.01 mol% or more, still more preferably 0.05 mol% or more, still more preferably 1.95 mol% or less, still more preferably 0.95 mol% or less, based on the entire monomer units of PVdF (a).

The PVdF (A) may further contain a non-fluorinated monomer unit (excluding the pentenoic acid unit represented by the formula (1)). Examples of the non-fluorinated monomer include ethylene and propylene.

The weight average molecular weight (in terms of polystyrene) of PVdF (a) is preferably 50000 ～ 3000000, more preferably 80000 or more, further preferably 100000 or more, particularly preferably 200000 or more, more preferably 2400000 or less, further preferably 2200000 or less, particularly preferably 2000000 or less, since an electrode mixture having a moderate viscosity and excellent coatability can be produced while ensuring sufficient adhesion to a metal foil. The weight average molecular weight can be determined by Gel Permeation Chromatography (GPC) using N, N-dimethylformamide as solvent at 50 ℃. The weight average molecular weight of PVdF (a) may be 1000000 or more.

The number average molecular weight (in terms of polystyrene) of PVdF (a) is preferably 20000 ～ 1500000, more preferably 40000 or more, still more preferably 70000 or more, particularly preferably 140000 or more, still more preferably 1400000 or less, still more preferably 1200000 or less, particularly preferably 1100000 or less, since an electrode mixture having moderate viscosity and excellent coatability can be prepared while ensuring sufficient adhesion to a metal foil. The number average molecular weight can be determined by Gel Permeation Chromatography (GPC) using dimethylformamide as a solvent.

In the case of using the conventional PVdF as a binder, the following tendency is observed: the higher the molecular weight of PVdF, the higher the adhesion of the electrode material layer to the metal foil. On the other hand, in the case of the conventional PVdF, the following tendency is observed: the higher the molecular weight, the higher the viscosity of the electrode mixture, and the lower the coatability of the electrode mixture. Since the composition of the present invention contains PVdF (a), an electrode material layer exhibiting sufficient adhesion to a metal foil can be formed, and an electrode mixture having a moderate viscosity and excellent coatability can be prepared. When PVdF (a) contained in the composition of the invention has a weight average molecular weight or a number average molecular weight within the above range, both high adhesion between the electrode material layer and the metal foil and excellent coatability of the electrode mixture can be achieved at a higher level.

For example, by using the composition of the present invention as a binder, an electrode material layer exhibiting sufficient adhesion to a metal foil can be formed without changing the ratio of the amount of the powder electrode material to the amount of the binder, even when the amount of the nonaqueous solvent is reduced by increasing the amount of the powder electrode material and the binder in the electrode material compared with the conventional electrode material, and an electrode material layer having the same viscosity as the conventional electrode material can be prepared. Therefore, by using the composition of the present invention as a binder, an electrode exhibiting excellent characteristics can be formed, and an improvement in electrode productivity and reduction in cost required for a nonaqueous solvent can be achieved.

The solution viscosity of PVdF (a) is preferably 10 to 4000mpa·s, more preferably 50mpa·s or more, still more preferably 100mpa·s or more, particularly preferably 150mpa·s or more, still more preferably 3000mpa·s or less, still more preferably 2000mpa·s or less, particularly preferably 1500mpa·s or less, since an electrode mixture having suitable viscosity and excellent coatability, which can further improve adhesion to a metal foil, can be prepared. The solution viscosity is the viscosity of an N-methyl-2-pyrrolidone (NMP) solution containing 5 mass% PVdF. The viscosity of NMP solutions can be measured using a type B viscometer at 25 ℃. The solution viscosity of PVdF (A) may be 400 mPas or more.

The electrode mixture having a moderate viscosity is excellent in coating property and easy in liquid transportation, and the powder electrode material can obtain good dispersibility in the electrode mixture. Therefore, the electrode mixture having a proper viscosity can shorten the time required for liquid transport, the powder electrode material is less likely to aggregate during liquid transport or storage, and the viscosity after liquid transport or storage can be easily readjusted. In addition, the electrode mixture having a moderate viscosity easily provides a moderate shearing force to the electrode mixture by stirring, and therefore, the powder electrode material can be easily dispersed in a nonaqueous solvent. When PVdF (a) contained in the composition of the invention has a solution viscosity in the above range, an electrode mixture having a proper viscosity and excellent coatability can be more easily prepared.

The melting point of PVdF (A) is preferably 100℃to 240 ℃. The above melting point can be obtained as follows: the temperature was increased at a rate of 10℃per minute using a Differential Scanning Calorimeter (DSC) apparatus, and the temperature was obtained as a temperature corresponding to the maximum value of the melting heat curve at that time.

The storage modulus of PVdF (A) at 30℃is preferably 2000MPa or less, more preferably 1800MPa or less.

The storage modulus of PVdF (A) at 60℃is preferably 1500MPa or less, more preferably 1300MPa or less.

The storage modulus of PVdF (A) at 30℃is preferably 1000MPa or more, more preferably 1100MPa or more.

The storage modulus at 60℃of PVdF (A) is preferably 600MPa or more, more preferably 700MPa or more.

When the storage modulus of PVdF (a) at 30 ℃ or 60 ℃ is within the above range, an electrode having improved flexibility and being less likely to break when used as a binder can be easily formed.

The storage modulus is a measured value obtained as follows: for a sample having a length of 30mm, a width of 5mm and a thickness of 50 μm to 100 μm, measurement values at 30℃and 60℃were measured by dynamic viscoelasticity under conditions of a stretching mode, a grip width of 20mm, a measurement temperature of-30℃to 160℃and a heating rate of 2℃per minute and a frequency of 1Hz using a dynamic viscoelasticity apparatus DVA220 manufactured by IT Keisokuseigyo.

The assay samples can be prepared as follows: for example, a measurement sample can be produced by dissolving PVdF (A) in N-methyl-2-pyrrolidone (NMP) to a concentration of 10 to 20% by mass, pouring the solution onto a glass plate, drying at 100℃for 12 hours, and further drying at 100℃for 12 hours under vacuum, and cutting the obtained film having a thickness of 50 to 100 μm into a film having a length of 30mm and a width of 5 mm.

The distribution state of the pentenoic acid units in the main chain of PVdF (a) is not particularly limited, and it is preferable that the pentenoic acid units are distributed as randomly as possible because the electrolyte swelling resistance, the adhesion to a metal foil, and the flexibility are further improved and the heat resistance is also improved. The "fraction of the randomly distributed pentenoic acid units" representing the ratio of the randomly distributed pentenoic acid units to the total number of pentenoic acid units in PVdF (a) is preferably 5% or more, more preferably 10% or more, still more preferably 15% or more.

The fraction of the randomly distributed pentenoic acid units can be calculated according to the following formula.

(Fraction (%)) = (average number of pentenoic acid units (%))/(average total number of pentenoic acid units (%)) ×100

In the formula, the pentenoic acid unit alignment can be determined by, for example, ¹⁹ F-NMR measurement and ¹ H-NMR measurement.

The pentenoic acid unit arrangement is an isolated pentenoic acid unit comprised between 2 VdF units, the greater the number of pentenoic acid unit arrangements, the higher the fraction of randomly distributed pentenoic acid units. In the case of a completely random distribution of pentenoic acid units, the average number of pentenoic acid units arranged is equal to the average total number of pentenoic acid units, and therefore the fraction of the pentenoic acid units distributed randomly is 100%.

PVdF (a) can be produced by polymerizing a monomer mixture containing at least VdF and pentenoic acid represented by formula (1). As the polymerization method, suspension polymerization, emulsion polymerization, solution polymerization, and the like can be used, and aqueous suspension polymerization and emulsion polymerization are preferable in view of ease of post-treatment and the like.

In the above polymerization, a polymerization initiator, a surfactant, a chain transfer agent, and a solvent may be used, and conventionally known ones may be used, respectively. As the polymerization initiator, an oil-soluble radical polymerization initiator or a water-soluble radical polymerization initiator can be used.

As the oil-soluble radical polymerization initiator, known oil-soluble peroxides can be used, and the following can be exemplified as representative examples:

Dialkyl peroxycarbonates such as di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, and di-sec-butyl peroxydicarbonate;

Peroxyesters such as t-butyl peroxyisobutyrate, t-butyl peroxypivalate, t-hexyl peroxyde (2-ethylhexanoate), t-butyl peroxyde (2-ethylhexanoate), and 1, 3-tetramethylbutyl peroxyde 2-ethylhexanoate;

dialkyl peroxides such as di-t-butyl peroxide;

Di [ fluoro (or fluoro chloro) acyl ] peroxides; etc.

Examples of the di [ fluoro (or fluorochloroacyl ] peroxides include diacyl peroxides represented by [ (RfCOO) - ] ₂ (Rf is a perfluoroalkyl group, ω -hydroperfluoroalkyl group, or fluorochloroalkyl group).

Examples of the di [ fluoro (or fluorochloroacyl ] peroxides include di (ω -hydro-dodecafluoroheptanoyl) peroxide, di (ω -hydro-tetradecahaloyl) peroxide, di (ω -hydro-hexadecahaloyl) peroxide, di (perfluorobutanoyl) peroxide, di (perfluoropentanoyl) peroxide, di (perfluorohexanoyl) peroxide, di (perfluoroheptanoyl) peroxide, di (perfluorooctanoyl) peroxide, di (perfluorononanoyl) peroxide, di (ω -chloro-hexafluorobutanoyl) peroxide, di (ω -chloro-decafluorohexanoyl) peroxide, di (ω -chloro-tetradecanoyl) peroxide, ω -hydro-dodecafluoroheptanoyl- ω -hexadecanoyl-peroxide, ω -chloro-hexafluoroheptanoyl-peroxide, ω -hydrododecafluoroheptanoyl-perfluoro-peroxide, di (dichloro-penta-fluoroheptanoyl) peroxide, di (dichloro-penta-fluorobutanoyl) peroxide, di (trichlorooctanoyl) peroxide, di (chloro-dodecanoyl) peroxide, and di (chloro-dodecanoyl) peroxide.

The water-soluble radical polymerization initiator may be a known water-soluble peroxide, and examples thereof include ammonium salts such as persulfuric acid, perboric acid, perchloric acid, perphosphoric acid, and percarbonic acid, organic peroxides such as potassium salt, sodium salt, disuccinic acid peroxide, and dipentaerythritol peroxide, t-butyl peroxymaleate, and t-butyl hydroperoxide. The reducing agent such as sulfite may be used in combination with the peroxide in an amount of 0.1 to 20 times the amount of the peroxide.

As the surfactant, a known surfactant can be used, and for example, a nonionic surfactant, an anionic surfactant, a cationic surfactant, and the like can be used. Among them, the fluorinated anionic surfactant is preferable, and the fluorinated anionic surfactant having 4 to 20 carbon atoms, which may be linear or branched, and which may or may not have an ether bond (i.e., may have an oxygen atom interposed between carbon atoms), is more preferable. The amount of the surfactant to be added (relative to the solvent) is preferably 50ppm to 5000ppm.

Examples of the chain transfer agent include: hydrocarbons such as ethane, isopentane, n-hexane, and cyclohexane; aromatic compounds such as toluene and xylene; ketones such as acetone; acetate esters such as ethyl acetate and butyl acetate; alcohols such as methanol and ethanol; mercaptans such as methyl mercaptan; halogenated hydrocarbons such as carbon tetrachloride, chloroform, methylene chloride and methyl chloride; etc. The amount of the chain transfer agent added may vary depending on the size of the chain transfer constant of the chain transfer agent, and is usually in the range of 0.01 to 20 mass% relative to the solvent.

Examples of the solvent include water, a mixed solvent of water and alcohol, and the like.

In the polymerization such as suspension polymerization, a fluorine-based solvent may be used in addition to water. Examples of the fluorine-based solvent include hydrochlorofluoroalkanes such as CH₃CClF₂、CH₃CCl₂F、CF₃CF₂CCl₂H、CF₂ClCF₂CFHCl; chlorofluoroalkanes such as CF ₂ClCFClCF₂CF₃、CF₃CFClCFClCF₃; preferred among them are hydrofluoroalkanes such as perfluorobutane 、CF₃CF₂CF₂CF₃、CF₃CF₂CF₂CF₂CF₃、CF₃CF₂CF₂CF₂CF₂CF₃, hydrofluorocarbons such as ;CF₂HCF₂CF₂CF₂H、CF₃CFHCF₂CF₂CF₃、CF₃CF₂CF₂CF₂CF₂H、CF₃CF₂CFHCF₂CF₃、CF₃CFHCFHCF₂CF₃、CF₂HCF₂CF₂CF₂CF₂H、CF₂HCFHCF₂CF₂CF₃、CF₃CF₂CF₂CF₂CF₂CF₂H、CF₃CH(CF₃)CF₃CF₂CF₃、CF₃CF(CF₃)CFHCF₂CF₃、CF₃CF(CF₃)CFHCFHCF₃、CF₃CH(CF₃)CFHCF₂CF₃、CF₂HCF₂CF₂CF₂CF₂CF₂H、CF₃CF₂CF₂CF₂CH₂CH₃、CF₃CH₂CF₂CH₃, and (perfluoroalkyl) alkyl ethers such as ;F(CF₂)₄OCH₃、F(CF₂)₄OC₂H₅、(CF₃)₂CFOCH₃、F(CF₂)₃OCH₃, and hydrofluoroalkyl ethers such as ;CF₃CH₂OCF₂CHF₂、CHF₂CF₂CH₂OCF₂CHF₂、CF₃CF₂CH₂OCF₂CHF₂. The amount of the fluorine-based solvent to be used is preferably 10 to 100% by mass based on the solvent in terms of suspension property and economy.

The polymerization temperature and polymerization pressure are appropriately determined according to the kind and amount of the solvent used and other polymerization conditions such as vapor pressure, polymerization temperature, etc.

From the viewpoint of enabling efficient production of PVdF (A), it is also preferable to polymerize a monomer mixture containing at least VdF and pentenoic acid represented by formula (1) under conditions in which VdF reaches a supercritical state. The critical temperature of VdF is 30.1 ℃ and the critical pressure is 4.38MPa.

From the viewpoint of enabling efficient production of PVdF (a), it is also preferable to supply the monomer mixture to the reactor so that the density of the monomer mixture in the reactor is sufficiently increased. The density of the monomer mixture in the reactor at the polymerization initiation temperature is preferably 0.20g/cm ³ or more, more preferably 0.23g/cm ³ or more, still more preferably 0.25g/cm ³ or more, and the upper limit is not particularly limited, but if the density is too high, the pressure in the reactor tends to be too large due to the temperature change in the reactor, so that it is preferably 0.70g/cm ³ or less from the viewpoint of safety in production. The density of the monomer mixture in the reactor can be obtained by dividing the supply amount (g) of the monomer mixture supplied to the reactor by the internal volume (cm ³) of the reactor minus the volume of water (cm ³).

In suspension polymerization using water as a dispersion medium, a suspending agent such as methylcellulose, methoxymethylcellulose, propoxylated methylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyoxyethylene, or gelatin may be added in an amount of 0.005 to 1.0 mass%, preferably 0.01 to 0.4 mass%, based on water.

As the polymerization initiator in this case, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di-n-heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, bis (chlorofluoroacyl) peroxide, bis (perfluoroacyl) peroxide, and the like can be used. The amount thereof is preferably 0.1 to 5% by mass relative to the total amount of the monomers.

Further, a chain transfer agent such as ethyl acetate, methyl acetate, acetone, methanol, ethanol, n-propanol, acetaldehyde, propionaldehyde, ethyl propionate, carbon tetrachloride, etc. may be added to adjust the polymerization degree of the obtained polymer. The amount thereof is usually 0.1 to 5% by mass, preferably 0.5 to 3% by mass relative to the total amount of the monomers.

The total amount of the monomers is as follows: the mass ratio of water is 1:1 to 1: 10. preferably 1: 2-1: 5.

PVdF (a) is used to prepare a composition containing PVdF (a) and VdF polymer (B) (excluding PVdF (a)). The composition thus produced can form an electrode excellent in electrolyte swelling resistance, adhesion to a metal foil, and flexibility, and further can provide an electrode mixture which is less likely to increase in viscosity, as in the composition of the present invention. The composition thus prepared can have the same constitution as the composition of the present invention and can be used for the same purpose.

The composition of the present invention contains VdF polymer (B) (excluding PVdF (a)) in addition to PVdF (a).

The VdF polymer (B) preferably contains VdF units and units based on a monomer copolymerizable with VdF (excluding VdF and pentenoic acid represented by formula (1)) from the viewpoint of forming an electrode having more excellent electrolyte swelling resistance and adhesion to a metal foil, and more excellent flexibility, and further, being capable of obtaining an electrode mixture having less tendency to rise in viscosity. As the VdF polymer (B), a polymer containing VdF units and units based on a monomer copolymerizable with VdF (excluding VdF and pentenoic acid represented by formula (1)), and VdF homopolymer may be used. Further, as the VdF polymer (B), 2 or more kinds of polymers containing VdF units and units based on a monomer copolymerizable with VdF excluding VdF and pentenoic acid represented by formula (1) may be used.

Examples of the monomer copolymerizable with VdF include fluorinated monomers and non-fluorinated monomers.

The fluorinated monomer (excluding VdF) is preferably at least one selected from the group consisting of Tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), perfluoroalkyl ethylene, 2, 3-tetrafluoropropene, and trans-1, 3-tetrafluoropropene, more preferably at least one selected from the group consisting of TFE, CTFE, and HFP, further preferably at least one selected from the group consisting of TFE and HFP, and particularly preferably TFE, from the standpoint of obtaining an electrode mixture that has further improved adhesion to a metal foil, electrolyte swelling resistance, and flexibility, and is less prone to increase in viscosity.

The fluorinated monomer units (excluding VdF units) may or may not have polar groups.

Examples of the non-fluorinated monomer include a non-fluorinated monomer having no polar group such as ethylene and propylene, a non-fluorinated monomer having a polar group (hereinafter, sometimes referred to as a polar group-containing monomer), and the like. When a substance having a polar group is used as the non-fluorinated monomer, the polar group is introduced into the VdF polymer (B), whereby the adhesion of the composition to the metal foil is further improved.

The VdF polymer (B) may have a polar group, whereby the adhesion of the composition to the metal foil is further improved. The polar group is not particularly limited as long as it is a functional group having a polarity, and is preferably at least one selected from the group consisting of a carbonyl group-containing group, an epoxy group, a hydroxyl group, a sulfonic acid group, a sulfuric acid group, a phosphoric acid group, an amino group, an amide group, and an alkoxy group, more preferably at least one selected from the group consisting of a carbonyl group-containing group, an epoxy group, and a hydroxyl group, and even more preferably a carbonyl group-containing group, from the viewpoint of further improving the adhesion of the composition to a metal foil. The hydroxyl group does not include a hydroxyl group constituting a part of the carbonyl group-containing group. The amino group refers to a 1-valent functional group obtained by removing hydrogen from ammonia, a primary amine, or a secondary amine.

The above carbonyl group-containing group means a functional group having a carbonyl group (-C (=o) -). The carbonyl group-containing group is preferably represented by the general formula: -COOR (R represents a hydrogen atom, an alkyl group or a hydroxyalkyl group) or a carboxylic anhydride group, more preferably of the general formula: -a group represented by COOR. The number of carbon atoms of the alkyl group and the hydroxyalkyl group is preferably 1 to 16, more preferably 1 to 6, and still more preferably 1 to 3. As the general formula: specifically, the group represented by COOR includes -COOCH₂CH₂OH、-COOCH₂CH(CH₃)OH、-COOCH(CH₃)CH₂OH、-COOH、-COOCH₃、-COOC₂H₅. In the general formula: in the case where the group represented by-COOR is-COOH or includes-COOH, the-COOH may be a carboxylate such as a metal carboxylate or an ammonium carboxylate.

The carbonyl group-containing group may be represented by the general formula: X-COOR (X represents an atomic group having a molecular weight of 500 or less, wherein the main chain is composed of 1 to 20 atoms, and R represents a hydrogen atom, an alkyl group or a hydroxyalkyl group). The number of carbon atoms of the alkyl group and the hydroxyalkyl group is preferably 1 to 16, more preferably 1 to 6, and still more preferably 1 to 3.

The amide group is preferably represented by the general formula: -CO-NRR '(R and R' independently represent a hydrogen atom or a substituted or unsubstituted alkyl group), or a group represented by the general formula: -CO-NR "- (R" represents a bond represented by a hydrogen atom, a substituted or unsubstituted alkyl group or a substituted or unsubstituted phenyl group).

The polar group may be introduced into the VdF polymer (B) by polymerizing VdF and a polar group-containing monomer, or may be introduced into the VdF polymer (B) by reacting the VdF polymer with a compound having the polar group, and VdF and a polar group-containing monomer are preferably polymerized from the viewpoint of productivity.

The polar group-containing monomer may be: hydroxyalkyl (meth) acrylates such as hydroxyethyl acrylate and 2-hydroxypropyl acrylate; alkylidene malonates such as dimethyl methine malonate; vinyl carboxyalkyl ethers such as vinyl carboxymethyl ether and vinyl carboxyethyl ether; carboxyalkyl (meth) acrylates such as 2-carboxyethyl acrylate and 2-carboxyethyl methacrylate; (meth) acryloyloxyalkyl dicarboxylic acid esters such as acryloyloxyethyl succinate, acryloyloxypropyl succinate, methacryloyloxyethyl succinate, acryloyloxyethyl phthalate and methacryloyloxyethyl phthalate; monoesters of unsaturated dibasic acids such as monomethyl maleate, monoethyl maleate, monomethyl citraconate and monoethyl citraconate; formula (2):

[ chemical 2]

(Wherein R ¹～R³ independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms R ⁴ represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms Y ¹ represents an inorganic cation and/or an organic cation.) a monomer (2) (wherein pentenoic acid represented by the formula (1) is not included); etc.

The VdF polymer (B) preferably contains a unit based on the monomer (2) represented by the formula (2) as the above polar group-containing monomer.

In formula (2), Y ¹ represents an inorganic cation and/or an organic cation. Examples of the inorganic cation include a cation such as H, li, na, K, mg, ca, al, fe. Examples of the organic cation include NH₄、NH₃R⁵、NH₂R⁵ ₂、NHR⁵ ₃、NR⁵ ₄(R⁵ independently represent an alkyl group having 1 to 4 carbon atoms) and the like. Y ¹ is preferably H, li, na, K, mg, ca, al, NH ₄, more preferably H, li, na, K, mg, al, NH ₄, further preferably H, li, al, NH ₄, and particularly preferably H. For convenience, specific examples of the inorganic cations and the organic cations are omitted from the description by reference numerals and valence.

In the formula (2), R ¹～R³ independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. The above-mentioned hydrocarbon group is a 1-valent hydrocarbon group. The number of carbon atoms of the hydrocarbon group is preferably 4 or less. Examples of the hydrocarbon group include an alkyl group, an alkenyl group, an alkynyl group, and the like having the above carbon number, and methyl or ethyl is preferable. R ¹ and R ² are independently preferably a hydrogen atom, a methyl group or an ethyl group, and R ³ is preferably a hydrogen atom or a methyl group.

In the formula (2), R ⁴ represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. The above-mentioned hydrocarbon group is a 2-valent hydrocarbon group. The number of carbon atoms of the hydrocarbon group is preferably 4 or less. The hydrocarbon group includes an alkylene group and an alkenylene group having the above carbon number, and among them, at least one selected from the group consisting of a methylene group, an ethylene group, an ethylidene group, a propylidene group and an isopropylidene group is preferable, and a methylene group is more preferable.

The monomer (2) is preferably at least one selected from the group consisting of (meth) acrylic acid and salts thereof, vinylacetic acid (3-butenoic acid) and salts thereof, 3-pentenoic acid and salts thereof, 3-hexenoic acid and salts thereof, 4-heptenoic acid and salts thereof, and 5-hexenoic acid and salts thereof.

When the VdF polymer is reacted with the compound having a polar group to introduce the polar group into the VdF polymer (B), the compound having a polar group may be a silane coupling agent or a titanate coupling agent having a group reactive with the VdF polymer and a hydrolyzable group. The hydrolyzable group is preferably an alkoxy group. In the case of using a coupling agent, the VdF polymer dissolved or swollen in a solvent is reacted with the VdF polymer, thereby enabling addition to the VdF polymer.

As the VdF polymer (B), vdF polymer (B) obtained by partially dehydrofluorinating VdF polymer with a base and then further reacting the partially dehydrofluorinated VdF polymer with an oxidizing agent may be used. Examples of the oxidizing agent include hydrogen peroxide, hypochlorite, palladium halide, chromium halide, alkali metal permanganate, peroxide, alkyl peroxide, and alkyl persulfate.

The content of the polar group-containing monomer unit of the VdF polymer (B) is preferably 0.001 mol% to 8.0 mol%, more preferably 0.01 mol% to 5.0 mol%, and even more preferably 0.30 mol% to 3.0 mol% with respect to the entire monomer units of the VdF polymer (B), from the viewpoint of further improving the adhesion of the composition to the metal foil.

In the present invention, the composition of the VdF polymer (B) can be measured by ¹⁹ F-NMR measurement, for example. The content of the polar group-containing monomer unit in the VdF polymer (B) when the VdF polymer (B) contains the polar group-containing monomer unit can be measured by acid-base titration of an acid group, for example, when the polar group is an acid group such as a carboxylic acid.

As the VdF polymer (B), a polymer containing VdF units and fluorinated monomer units (VdF units are not included therein) is preferable. In addition to these monomer units, the VdF polymer (B) may contain a non-fluorinated monomer unit such as a polar group-containing monomer unit.

The content of VdF units in the VdF polymer (B) is preferably 57.0 mol% to 99.9 mol%, more preferably 60.0 mol% or more, still more preferably 63.0 mol% or more, and still more preferably 99.5 mol% or less, based on the total monomer units in the VdF polymer (B).

The content of the fluorinated monomer unit of the VdF polymer (B) is preferably 0.1 mol% to 43.0 mol%, more preferably 0.5 mol% or more, still more preferably 40.0 mol% or less, and still more preferably 37.0 mol% or less, based on the total monomer units of the VdF polymer (B).

VdF polymer (B) also preferably contains a relatively small amount of VdF units and a relatively large amount of fluorinated monomer units. For example, the content of VdF units is preferably 57.0 mol% or more, more preferably 60.0 mol% or more, still more preferably 63.0 mol% or more, preferably 95.0 mol% or less, more preferably 90.0 mol% or less, still more preferably 85.0 mol% or less, based on the total monomer units of VdF polymer (B). The content of the fluorinated monomer unit is preferably 5.0 mol% or more, more preferably 8.0 mol% or more, particularly preferably 10.0 mol% or more, most preferably 15.0 mol% or more, preferably 43.0 mol% or less, more preferably 40.0 mol% or less, further preferably 38.0 mol% or less, particularly preferably 37.0 mol% or less, based on the total monomer units of the VdF polymer (B).

The VdF polymer (B) containing a relatively small amount of VdF units and a relatively large amount of fluorinated monomer units also preferably further contains polar group-containing monomer units. The content of the polar group-containing monomer unit is preferably 0.05 to 2.0 mol%, more preferably 0.10 mol% or more, still more preferably 0.25 mol% or more, particularly preferably 0.40 mol% or more, and still more preferably 1.5 mol% or less, based on the total monomer units of the VdF polymer (B).

The VdF polymer (B) also preferably contains a relatively large amount of VdF units and a relatively small amount of monomer units copolymerizable with VdF. For example, the content of VdF units is preferably 92.0 mol% to 99.9 mol%, more preferably 95.0 mol% or more, and still more preferably 99.5 mol% or less, based on the total monomer units of VdF polymer (B). The content of the monomer unit copolymerizable with VdF is preferably 0.10 mol% to 8.0 mol%, more preferably 0.50 mol% or more, and still more preferably 5.0 mol% or less, with respect to the entire monomer units of VdF polymer (B).

The weight average molecular weight (in terms of polystyrene) of the VdF polymer (B) is preferably 50000 ～ 3000000, more preferably 80000 or more, still more preferably 100000 or more, particularly preferably 200000 or more, still more preferably 2400000 or less, still more preferably 2200000 or less, particularly preferably 2000000 or less. The weight average molecular weight can be measured by Gel Permeation Chromatography (GPC) using dimethylformamide as a solvent.

The number average molecular weight (in terms of polystyrene) of the VdF polymer (B) is preferably 20000 ～ 1500000, more preferably 40000 or more, still more preferably 70000 or more, particularly preferably 140000 or more, still more preferably 1400000 or less, still more preferably 1200000 or less, and particularly preferably 1100000 or less. The number average molecular weight can be measured by Gel Permeation Chromatography (GPC) using dimethylformamide as a solvent.

The solution viscosity of the VdF polymer (B) is preferably 10mpa·s to 4000mpa·s, more preferably 50mpa·s or more, still more preferably 100mpa·s or more, particularly preferably 150mpa·s or more, still more preferably 3000mpa·s or less, still more preferably 2000mpa·s or less, particularly preferably 1500mpa·s or less. The solution viscosity is the viscosity of an N-methyl-2-pyrrolidone (NMP) solution containing 5 mass% of VdF polymer. The viscosity of NMP solutions can be measured using a type B viscometer at 25 ℃.

The melting point of the VdF polymer (B) is preferably 100℃to 240 ℃. The melting point can be determined as follows: the temperature was increased at a rate of 10℃per minute using a Differential Scanning Calorimeter (DSC) apparatus, and the temperature was obtained as a temperature corresponding to the maximum value of the melting heat curve at that time.

The VdF polymer (B) preferably has a storage modulus at 30℃of 1100MPa or less and a storage modulus at 60℃of 500MPa or less. The flexibility is further improved when the storage modulus of the VdF polymer (B) at 30 ℃ is 1100MPa or less and the storage modulus at 60 ℃ is 500MPa or less.

The storage modulus of the VdF polymer (B) at 30℃is more preferably 800MPa or less, still more preferably 600MPa or less.

The storage modulus of the VdF polymer (B) at 60℃is more preferably 350MPa or less.

The storage modulus of the VdF polymer (B) at 30℃is preferably 100MPa or more, more preferably 150MPa or more, and still more preferably 200MPa or more.

The storage modulus of the VdF polymer (B) at 60℃is preferably 50MPa or more, more preferably 80MPa or more, and still more preferably 130MPa or more.

The storage modulus of the VdF polymer (B) can be measured by the same method as that of PVdF (a).

Examples of the VdF polymer (B) include VdF/TFE copolymer, vdF/HFP copolymer, vdF/2, 3-tetrafluoropropene copolymer, vdF/TFE/HFP copolymer, vdF/TFE/2, 3-tetrafluoropropene copolymer, vdF/TFE/(meth) acrylic acid copolymer, vdF/HFP/(meth) acrylic acid copolymer, vdF/CTFE/TFE copolymer, vdF/TFE/3-butenoic acid copolymer, vdF/TFE/HFP/(meth) acrylic acid copolymer, vdF/TFE/HFP/3-butenoic acid copolymer, vdF/TFE/2-carboxyethyl acrylate copolymer, vdF/TFE/HFP/2-carboxyethyl acrylate copolymer, vdF/TFE/succinoxyethyl acrylate copolymer, vdF/TFE/HFP/succinoxyethyl acrylate copolymer, and the like.

The VdF polymer (B) is preferably at least one selected from the group consisting of VdF/TFE copolymer, vdF/HFP copolymer, vdF/2, 3-tetrafluoropropene copolymer, vdF/TFE/HFP copolymer, vdF/TFE/2, 3-tetrafluoropropene copolymer, vdF/TFE/(meth) acrylic acid copolymer, vdF/HFP/(meth) acrylic acid copolymer, vdF/CTFE copolymer, and VdF/CTFE/TFE copolymer.

The VdF/TFE copolymer contains VdF units and TFE units. The content of the VdF unit is preferably 50 mol% to 95 mol%, more preferably 55 mol% or more, still more preferably 60 mol% or more, still more preferably 92 mol% or less, still more preferably 89 mol% or less, based on the total monomer units of the VdF/TFE copolymer. The content of TFE units is preferably 50 to 5 mol%, more preferably 45 mol% or less, still more preferably 40 mol% or less, still more preferably 8 mol% or more, still more preferably 11 mol% or more, based on the total monomer units of the VdF/TFE copolymer.

In addition to the VdF units and TFE units, the VdF/TFE copolymer may also comprise units based on monomers copolymerizable with VdF and TFE, excluding VdF, TFE and pentenoic acid represented by formula (1). From the viewpoint of resistance to swelling with an electrolyte, the content of the unit based on the monomer copolymerizable with VdF and TFE is preferably 3.0 mol% or less with respect to the total monomer units of the VdF/TFE copolymer.

Examples of the monomer copolymerizable with VdF and TFE include the fluorinated monomer described above, the non-fluorinated monomer described above, and the like. As the monomer copolymerizable with VdF and TFE, among them, at least one selected from the group consisting of a fluorinated monomer and a polar group-containing monomer is preferable, and at least one selected from the group consisting of HFP, 2, 3-tetrafluoropropene, and monomer (2) (wherein pentenoic acid represented by formula (1) is not included) is more preferable.

The weight average molecular weight (in terms of polystyrene) of the VdF/TFE copolymer is preferably 50000 ～ 2000000, more preferably 80000 ～ 1700000, and even more preferably 100000 ～ 1500000.

The number average molecular weight (in terms of polystyrene) of the VdF/TFE copolymer is 35000 ～ 1400000, more preferably 40000 ～ 1300000, and still more preferably 50000 ～ 1200000.

The VdF/HFP copolymer contains VdF units and HFP units. The content of VdF units is preferably 80 to 98 mol%, more preferably 83 mol% or more, still more preferably 85 mol% or more, still more preferably 97 mol% or less, still more preferably 96 mol% or less, based on the total monomer units of the VdF/HFP copolymer. The content of the HFP unit is preferably 20 to 2 mol%, more preferably 17 mol% or less, still more preferably 15 mol% or less, still more preferably 3 mol% or more, still more preferably 4 mol% or more, based on the total monomer units of the VdF/HFP copolymer.

In addition to the VdF unit and HFP unit, the VdF/HFP copolymer may also contain a unit based on a monomer copolymerizable with VdF and HFP, excluding VdF, HFP and pentenoic acid represented by formula (1). From the viewpoint of resistance to swelling with an electrolyte, the content of the unit based on the monomer copolymerizable with VdF and HFP is preferably 3.0 mol% or less with respect to the total monomer units of the VdF/HFP copolymer.

Examples of the monomer copolymerizable with VdF and HFP include the fluorinated monomer described above, the non-fluorinated monomer described above, and the like. As the monomer copolymerizable with VdF and HFP, among them, at least one selected from the group consisting of a fluorinated monomer and a polar group-containing monomer is preferable, at least one selected from the group consisting of TFE, 2, 3-tetrafluoropropene, and monomer (2) (excluding pentenoic acid represented by formula (1)) is more preferable.

The weight average molecular weight (in terms of polystyrene) of the VdF/HFP copolymer is preferably 50000 ～ 2000000, more preferably 80000 ～ 1700000, and still more preferably 100000 ～ 1500000.

The number average molecular weight (in terms of polystyrene) of the VdF/HFP copolymer is preferably 35000 ～ 1400000, more preferably 40000 ～ 1300000, and still more preferably 50000 ～ 1200000.

The VdF/CTFE copolymer contains VdF units and CTFE units. The content of the VdF unit is preferably 80 to 98 mol%, more preferably 85 mol% or more, still more preferably 90 mol% or more, still more preferably 97.5 mol% or less, still more preferably 97 mol% or less, based on the total monomer units of the VdF/CTFE copolymer. The content of CTFE units is preferably 20 to 2 mol%, more preferably 15 mol% or less, still more preferably 10 mol% or less, still more preferably 2.5 mol% or more, still more preferably 3 mol% or more, based on the total monomer units of the VdF/CTFE copolymer.

In addition to the VdF units and CTFE units, the VdF/CTFE copolymer may also comprise units based on monomers copolymerizable with VdF and CTFE, excluding VdF, CTFE and pentenoic acid represented by formula (1). From the viewpoint of resistance to swelling with an electrolyte, the content of the unit based on the monomer copolymerizable with VdF and CTFE is preferably 3.0 mol% or less with respect to the total monomer units of the VdF/CTFE copolymer.

Examples of the monomer copolymerizable with VdF and CTFE include the fluorinated monomer described above, the non-fluorinated monomer described above, and the like. Among these monomers copolymerizable with VdF and CTFE, at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers is preferable, at least one selected from the group consisting of TFE, HFP, 2, 3-tetrafluoropropene and monomer (2) (wherein pentenoic acid represented by formula (1) is not included) is more preferable, and TFE is further preferable.

The weight average molecular weight (in terms of polystyrene) of the VdF/CTFE copolymer is preferably 50000 ～ 2000000, more preferably 80000 ～ 1700000, and still more preferably 100000 ～ 1500000.

The number average molecular weight (in terms of polystyrene) of the VdF/CTFE copolymer is preferably 35000 ～ 1400000, more preferably 40000 ～ 1300000, and still more preferably 50000 ～ 1200000.

The VdF/2, 3-tetrafluoropropene copolymer contains VdF units and 2, 3-tetrafluoropropene units. The content of the VdF unit is preferably 80 to 98 mol%, more preferably 85 mol% or more, still more preferably 90 mol% or more, still more preferably 97.5 mol% or less, still more preferably 97 mol% or less, based on the total monomer units of the VdF/2, 3-tetrafluoropropene copolymer. The content of the 2, 3-tetrafluoropropene unit is preferably 20 to 2 mol%, more preferably 15 mol% or less, still more preferably 10 mol% or less, still more preferably 2.5 mol% or more, still more preferably 3 mol% or more, based on the total monomer units of the VdF/2, 3-tetrafluoropropene copolymer.

In addition to the VdF units and the 2, 3-tetrafluoropropene units, the VdF/2, 3-tetrafluoropropene copolymer may also comprise a polymer based on a monomer copolymerizable with VdF and 2, 3-tetrafluoropropene (wherein VdF is excluded 2, 3-tetrafluoropropene and pentenoic acid represented by the formula (1). From the viewpoint of resistance to swelling with an electrolyte, the content of the unit based on the monomer copolymerizable with VdF and 2, 3-tetrafluoropropene is preferably 3.0 mol% or less with respect to the total monomer units of the VdF/2, 3-tetrafluoropropene copolymer.

Examples of the monomer copolymerizable with VdF and 2, 3-tetrafluoropropene include the fluorinated monomer described above, the non-fluorinated monomer described above, and the like. As the monomer copolymerizable with VdF and 2, 3-tetrafluoropropene, at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers is preferable, and at least one selected from the group consisting of TFE, HFP, 2, 3-tetrafluoropropene and monomer (2) (wherein pentenoic acid represented by formula (1) is not included) is more preferable.

The weight average molecular weight (in terms of polystyrene) of the VdF/2, 3-tetrafluoropropene copolymer is preferably 50000 ～ 2000000, more preferably 80000 ～ 1700000, and still more preferably 100000 ～ 1500000.

The number average molecular weight (in terms of polystyrene) of the VdF/2, 3-tetrafluoropropene copolymer is preferably 35000 ～ 1400000, more preferably 40000 ～ 1300000, and still more preferably 50000 ～ 1200000.

The mass ratio ((a)/(B)) of PVdF (a) to VdF polymer (B) in the composition is preferably 95/5 to 10/90, more preferably 90/10 or less, still more preferably 40/60 or more, still more preferably 45/55 or more, and particularly preferably 50/50 or more, from the viewpoint that an electrode mixture having further improved adhesion to a metal foil, electrolyte swelling resistance and flexibility, and less likely to increase in viscosity can be obtained. The mass ratio ((A)/(B)) may be 85/15 or more.

The compositions of the present invention may be suitable for use as binders. By using the above composition as a binder, an electrode having excellent electrolyte swelling resistance, adhesion to a metal foil, and excellent flexibility can be formed, and an electrode mixture having less tendency to increase in viscosity can be obtained.

The adhesive of the invention contains the composition. The binder of the present invention can form an electrode excellent in electrolyte swelling resistance, adhesion to a metal foil, and flexibility, and further can provide an electrode mixture which is less likely to increase in viscosity, because of the composition.

The binder of the present invention may contain polymers other than PVdF (a) and VdF polymer (B). Examples of the polymer other than the PVdF (a) and VdF polymer (B) include a fluoropolymer (excluding the PVdF (a) and VdF polymer (B)), a polymethacrylate, a polymethyl methacrylate, a polyacrylonitrile, a polyimide, a polyamide, a polyamideimide, a polycarbonate, a styrene rubber, a butadiene rubber, and the like.

The composition and the binder of the present invention may be suitably used as materials for forming a secondary battery. The composition and the binder of the present invention can form an electrode excellent in electrolyte swelling resistance and adhesion to a metal foil, and excellent in flexibility, and further can provide an electrode mixture which is less likely to increase in viscosity, and therefore are suitable as a binder for use in an electrode of a secondary battery. The composition and the binder of the present invention can also be used as a binder for a separator coating layer of a secondary battery. Further, by using the composition and the binder of the present invention, it is possible to produce a secondary battery which is difficult to increase the resistance value even when stored at high temperature and which can maintain a sufficient discharge capacity even when repeatedly charged and discharged.

The composition of the present invention may be a composition for a secondary battery. In the present invention, the composition for a secondary battery includes a composition used in a positive electrode, a negative electrode, and a separator of a secondary battery.

The binder of the present invention may be a binder for a secondary battery. In the present invention, the binder for a secondary battery includes binders used in the positive electrode, the negative electrode, and the separator of the secondary battery. The secondary battery is preferably a lithium ion secondary battery.

The composition or binder of the present invention may also constitute an electrode mixture together with a powder electrode material, water or a nonaqueous solvent. The secondary battery to which the composition or the binder of the present invention is applied is provided with: a positive electrode in which a positive electrode mixture is held by a positive electrode current collector, a negative electrode in which a negative electrode mixture is held by a negative electrode current collector, and an electrolyte.

The electrode mixture of the present invention contains the above composition or binder, a powder electrode material, and an aqueous or nonaqueous solvent. The electrode mixture of the present invention may be an electrode mixture for a secondary battery or an electrode mixture for a lithium ion secondary battery. The electrode mixture of the present invention, which contains the composition or the binder, can easily adjust the viscosity suitable for application to a current collector even when the composition or the binder is contained at a high concentration, and can form an electrode having excellent electrolyte swelling resistance, adhesion to a metal foil, and excellent flexibility, which is less likely to increase in viscosity even when stored for a long period of time. In addition, the electrode mixture of the present invention contains the composition or the binder, and thus, the composition or the binder can be easily adjusted to have a proper viscosity, and can maintain a proper viscosity for a long period of time, and can ensure sufficient adhesion to a metal foil (current collector) and sufficient holding power of a powder electrode material. Further, by using the electrode mixture of the present invention, a secondary battery can be produced which is less likely to increase the resistance value even when stored at high temperature and which can maintain a sufficient discharge capacity even when repeatedly charged and discharged.

The electrode mixture may be a positive electrode mixture used for producing a positive electrode or a negative electrode mixture used for producing a negative electrode, and is preferably a positive electrode mixture. The electrode material layer formed of the electrode mixture of the present invention may be a positive electrode material layer or a negative electrode material layer as long as the electrode material layer contains the composition or the binder and the powder electrode material.

The powder electrode material is a powder electrode material for a battery, and preferably contains an electrode active material. The electrode active material is divided into a positive electrode active material and a negative electrode active material. In the case of a lithium ion secondary battery, the positive electrode active material is not particularly limited as long as it can electrochemically store and release lithium ions, and is preferably a lithium composite oxide, more preferably a lithium transition metal composite oxide. As the positive electrode active material, a lithium-containing transition metal phosphate compound is also preferable. The positive electrode active material is preferably a material containing lithium and at least one transition metal, such as a lithium transition metal composite oxide and a lithium-containing transition metal phosphate compound.

The transition metal of the lithium transition metal composite oxide is preferably V, ti, cr, mn, fe, co, ni, cu, and specific examples of the lithium transition metal composite oxide include lithium cobalt composite oxides such as LiCoO ₂; lithium nickel composite oxides such as LiNiO ₂; lithium-manganese composite oxides such as LiMnO ₂、LiMn₂O₄、Li₂MnO₃; and those obtained by replacing a part of the transition metal atoms that are the main body of these lithium transition metal composite oxides with other metals such as Al, ti, V, cr, mn, fe, co, li, ni, cu, zn, mg, ga, zr, si. Examples of the substituted material include lithium-nickel-manganese composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-cobalt-manganese composite oxide, lithium-manganese-aluminum composite oxide, and lithium-titanium composite oxide, and more specifically, LiNi_0.5Mn_0.5O₂、LiNi_0.85Co_0.10Al_0.05O₂、LiNi_0.33Co_0.33Mn_0.33O₂、LiNi_0.5Mn_0.3Co_0.2O₂、LiNi_0.6Mn_0.2Co_0.2O₂、LiNi_0.8Mn_0.1Co_0.1O₂、LiMn_1.8Al_0.2O₄、LiMn_1.5Ni_0.5O₄、Li₄Ti₅O₁₂、LiNi_0.82Co_0.15Al_0.03O₂ and the like.

The transition metal of the lithium-containing transition metal phosphate compound is preferably V, ti, cr, mn, fe, co, ni, cu, and specific examples of the lithium-containing transition metal phosphate compound include iron phosphates such as LiFePO ₄、Li₃Fe₂(PO₄)₃、LiFeP₂O₇; cobalt phosphates such as LiCoPO ₄; and those obtained by replacing a part of the transition metal atoms that are the main body of these lithium transition metal phosphate compounds with other metals such as Al, ti, V, cr, mn, fe, co, li, ni, cu, zn, mg, ga, zr, nb, si.

Particularly, it is preferable from the viewpoints of high voltage, high energy density, charge-discharge cycle characteristics, and the like LiCoO₂、LiNiO₂、LiMn₂O₄、LiNi_0.82Co_0.15Al_0.03O₂、LiNi_0.33Mn_0.33Co_0.33O₂、LiNi_0.5Mn_0.3Co_0.2O₂、LiNi_0.6Mn_0.2Co_0.2O₂、LiNi_0.8Mn_0.1Co_0.1O₂、LiFePO₄.

The positive electrode active material obtained by this method may be used by attaching a substance having a composition different from that of the positive electrode active material constituting the main body to the surface of the positive electrode active material. Examples of the surface-adhering substance include oxides such as alumina, silica, titania, zirconia, magnesia, calcia, boria, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate; etc.

These surface-attached substances can be attached to the surface of the positive electrode active material by, for example, the following method: a method of dissolving or suspending in a solvent, impregnating the positive electrode active material with the solution, and drying the solution; a method in which a surface-attached substance precursor is dissolved or suspended in a solvent, impregnated and added to a positive electrode active material, and then reacted by heating or the like; a method of adding the active material to a positive electrode active material precursor and firing the active material precursor at the same time; etc.

The amount of the surface-adhering substance is preferably 0.1ppm or more, more preferably 1ppm or more, and still more preferably 10ppm or more, based on the mass of the positive electrode active material; the upper limit is preferably 20% or less, more preferably 10% or less, and still more preferably 5% or less, and the amount is used. The surface-attached matter can inhibit the oxidation reaction of the nonaqueous electrolyte on the surface of the positive electrode active material, thereby improving the battery life; however, in the case where the amount of the adhesive agent is too small, the effect thereof cannot be sufficiently exhibited; if the amount of the binder is excessive, the ingress and egress of lithium ions are hindered, and thus the resistance may be increased.

The shape of the positive electrode active material particles is preferably a block, polyhedron, sphere, elliptic sphere, plate, needle, column, or the like, which have been conventionally used, and it is preferable that the primary particles are aggregated to form secondary particles, and the secondary particles have a spherical or elliptic sphere shape. In general, in an electrochemical device, an active material in an electrode expands and contracts in response to charge and discharge, and thus deterioration such as destruction of the active material and disconnection of a conductive path due to stress is likely to occur. Therefore, a substance in which primary particles are aggregated to form secondary particles is preferable because it can alleviate the stress of expansion and contraction and prevent deterioration, compared with a single-particle active substance in which only primary particles are aggregated. In addition, since the orientation at the time of electrode molding is small in the case of spherical or ellipsoidal particles, the expansion and contraction of the electrode at the time of charge and discharge are small as compared with those of plate-like equiaxed orientation, and the electrode can be uniformly mixed with the conductive agent at the time of electrode production, which is preferable.

The tap density of the positive electrode active material is usually 1.3g/cm ³ or more, preferably 1.5g/cm ³ or more, more preferably 1.6g/cm ³ or more, and most preferably 1.7g/cm ³ or more. If the tap density of the positive electrode active material is less than the lower limit, the amount of the dispersion medium required for forming the positive electrode material layer increases, and the amount of the conductive agent or the binder required increases, so that the filling rate of the positive electrode active material in the positive electrode material layer may be limited and the battery capacity may be limited. By using a metal composite oxide powder having a high tap density, a high-density positive electrode material layer can be formed. The higher the tap density, the better the higher the specific upper limit, but if the higher the tap density, the rate of diffusion of lithium ions in the positive electrode material layer using the nonaqueous electrolyte as a medium becomes controlled, and the load characteristics may be easily lowered, so that the higher the tap density is usually 2.5g/cm ³ or less, preferably 2.4g/cm ³ or less.

The tap density of the positive electrode active material was obtained by passing the positive electrode active material through a 300 μm mesh sieve, dropping the sample into a 20cm ³ tap tank (TAPPING CELL) and filling the tank volume, and then oscillating the sample for 1000 cycles with a 10mm stroke length using a powder density measuring instrument (for example TAP DENSER manufactured by SEISHIN ENTERPRISE corporation), and the density was determined from the volume and the weight of the sample at this time, and the density was defined as tap density.

The median diameter d50 of the particles of the positive electrode active material (secondary particle diameter in the case where the primary particles are aggregated to form secondary particles) is usually 0.1 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, and most preferably 3 μm or more; usually 20 μm or less, preferably 18 μm or less, more preferably 16 μm or less, and most preferably 15 μm or less. If it is less than the above lower limit, a high bulk density product may not be obtained; if the upper limit is exceeded, the diffusion of lithium in the particles takes time, and therefore, there are problems such as degradation of battery performance, or streaking when the active material, the conductive agent, the binder, and the like are prepared into a slurry by using a solvent and applied in a film form in the production of a positive electrode of a battery. Here, by mixing two or more positive electrode active materials having different median diameters d50, the filling property at the time of positive electrode production can be further improved.

The median diameter d50 in the present invention was measured by a known laser diffraction/scattering particle size distribution measuring apparatus. LA-920 manufactured by HORIBA was used as a particle size distribution meter, and a 0.1 mass% aqueous solution of sodium hexametaphosphate was used as a dispersion medium for measurement, and after ultrasonic dispersion for 5 minutes, the measurement refractive index was set to 1.24 for measurement.

When the primary particles are aggregated to form secondary particles, the average primary particle diameter of the positive electrode active material is usually 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.08 μm or more, and most preferably 0.1 μm or more; usually 3 μm or less, preferably 2 μm or less, more preferably 1 μm or less, and most preferably 0.6 μm or less. If the upper limit is exceeded, it is difficult to form spherical secondary particles, which adversely affects the powder filling property or greatly reduces the specific surface area, and therefore the possibility of lowering the battery performance such as the output characteristics may be increased. Conversely, if the amount is less than the lower limit, the crystallinity may be underdeveloped, resulting in a problem such as poor charge/discharge reversibility. The primary particle size was measured by observation using a Scanning Electron Microscope (SEM). Specifically, in a photograph having a magnification of 10000, the longest value of a slice generated by the boundary line between the left and right of the primary particles with respect to the straight line in the horizontal direction is obtained for any 50 primary particles, and the average value is obtained, thereby obtaining the primary particle diameter.

The BET specific surface area of the positive electrode active material is 0.2m ²/g or more, preferably 0.3m ²/g or more, and more preferably 0.4m ²/g or more; it is 4.0m ²/g or less, preferably 2.5m ²/g or less, more preferably 1.5m ²/g or less. If the BET specific surface area is less than this range, the battery performance tends to be lowered; if the BET specific surface area is larger than this range, the tap density is less likely to increase, and the coatability in forming the positive electrode material layer may be more likely to be problematic.

The BET specific surface area is defined as follows: the BET specific surface area is defined by a value obtained by predrying a sample at 150℃for 30 minutes under a nitrogen flow using a surface area meter (for example, a fully automatic surface area measuring apparatus manufactured by Dagaku Kogyo Co., ltd.), and then measuring by a nitrogen adsorption BET single point method using a nitrogen helium mixed gas accurately adjusted to a relative pressure value of nitrogen with respect to atmospheric pressure of 0.3.

As a method for producing the positive electrode active material, a method common to a method for producing an inorganic compound is used. In particular, various methods are considered for producing spherical or ellipsoidal active materials, and examples thereof include the following methods: a method in which a transition metal raw material such as a transition metal nitrate or sulfate and a raw material of other element as necessary are dissolved or pulverized and dispersed in a solvent such as water, the pH is adjusted under stirring, a spherical precursor is produced and recovered, if necessary, dried, and then a Li source such as LiOH or Li ₂CO₃、LiNO₃ is added and fired at a high temperature to obtain an active material; a method in which a transition metal raw material such as a transition metal nitrate, sulfate, hydroxide, or oxide and a raw material of other element if necessary are dissolved or pulverized and dispersed in a solvent such as water, and dried and molded by a spray dryer or the like to obtain a spherical or oval spherical precursor, and a Li source such as LiOH or Li ₂CO₃、LiNO₃ is added thereto and fired at a high temperature to obtain an active material; and a method in which a transition metal raw material such as a transition metal nitrate, sulfate, hydroxide, or oxide, a Li source such as LiOH, li ₂CO₃、LiNO₃, or a raw material of other element if necessary, is dissolved or pulverized and dispersed in a solvent such as water, and the resultant mixture is dried and molded by a spray dryer or the like to form a spherical or elliptic spherical precursor, and the precursor is fired at a high temperature to obtain an active material; etc.

In the present invention, one kind of positive electrode active material powder may be used alone, or two or more kinds of positive electrode active material powders having different compositions or different powder physical properties may be used in combination in any combination and ratio.

The negative electrode active material is not particularly limited as long as it can electrochemically occlude and release lithium ions, and examples thereof include carbonaceous materials, metal oxides such as tin oxide and silicon oxide, metal composite oxides, lithium alloys such as lithium simple substance and lithium aluminum alloy, metals such as Sn and Si capable of forming alloys with lithium, and the like. One kind of them may be used alone, or two or more kinds thereof may be used in any combination and ratio. Among them, carbonaceous materials or lithium composite oxides are preferably used in view of safety.

The metal composite oxide is not particularly limited as long as it can store and release lithium, and preferably contains titanium and/or lithium as constituent components in view of high-current density charge/discharge characteristics.

The carbonaceous material is preferably selected from the following materials for the reason of good balance between initial irreversible capacity and high current density charge/discharge characteristics:

(1) Natural graphite;

(2) An artificial carbonaceous material and an artificial graphite material; carbonaceous materials { for example, natural graphite, coal-based coke, petroleum-based coke, coal-based pitch, petroleum-based pitch, or substances obtained by oxidizing these pitches, needle coke, pitch coke, and carbon materials obtained by partially graphitizing these, furnace black, acetylene black, organic matters such as pitch-based carbon fibers, carbonized organic matters (for example, coal tar pitch of soft pitch to hard pitch, or ethylene tar or the like by-produced during thermal decomposition of coal-based heavy oil such as carbonization liquefied oil, normal pressure residual oil, straight-run heavy oil of decompression residual oil, crude oil, naphtha, and the like, and aromatic hydrocarbons such as acenaphthylene, decacyclic olefin, anthracene, phenanthrene, and the like, N-ring compounds such as phenazine or acridine, S-ring compounds such as thiophene, dithiophene, polyphenyl such as biphenyl, terphenyl, etc.) polyvinyl chloride, polyvinyl alcohol, polyvinyl butyral, insoluble treated matters of these matters, organic polymers such as nitrogen-containing polyacrylonitrile and polypyrrole, organic polymers such as sulfur-containing polythiophene and polystyrene, natural polymers such as cellulose, lignin, mannan, polygalacturonic acid, chitosan and polysaccharides typified by sucrose, thermoplastic resins such as polyphenylene sulfide and polyphenylene ether, thermosetting resins such as furfuryl alcohol resins, phenol-formaldehyde resins and imide resins) and carbides thereof, or solutions obtained by dissolving carbonizable organic matters in low molecular organic solvents such as benzene, toluene, xylene, quinoline and N-hexane and carbonaceous materials obtained by heat-treating at 400 ℃ to 3200 ℃ for at least one time;

(3) The negative electrode material layer is made of at least two or more kinds of carbonaceous materials having different crystallinity and/or carbonaceous materials having interfaces at which the carbonaceous materials having different crystallinity are in contact;

(4) The negative electrode material layer is made of at least two or more kinds of carbonaceous materials having different orientations and/or carbonaceous materials having interfaces at which the carbonaceous materials having different orientations are in contact.

In order to increase the capacity of the obtained electrode, the content of the electrode active material (positive electrode active material or negative electrode active material) is preferably 40 mass% or more in the electrode mixture.

The above powder electrode material may further comprise a conductive agent. Examples of the conductive agent include carbon materials such as carbon black including acetylene black and ketjen black, carbon materials such as graphite, carbon fibers, carbon nanotubes, and carbon nanoprotrusions.

The ratio of the powder components (active material and conductive agent) in the electrode mixture to the above composition or binder is generally 80 by mass ratio: 20 to 99.5: about 0.5, can be determined in consideration of the retention of the powder component, the adhesion to the current collector, and the conductivity of the electrode.

In the case of such a mixing ratio, the composition or binder cannot completely fill the gaps between the powder components in the electrode material layer formed on the current collector; when a liquid capable of satisfactorily dissolving or dispersing the composition or binder is used as the solvent, the composition or binder is uniformly dispersed and formed into a mesh shape in the electrode material layer after drying, and the powder component can be satisfactorily maintained, which is preferable.

The liquid may be water or a nonaqueous solvent. Examples of the nonaqueous solvent include nitrogen-containing organic solvents such as N-methyl-2-pyrrolidone, N-dimethylacetamide, and dimethylformamide; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone; ester solvents such as ethyl acetate and butyl acetate; ether solvents such as tetrahydrofuran and dioxane; and low-boiling point general-purpose organic solvents such as mixed solvents thereof.

Among them, the liquid is preferably at least one selected from the group consisting of N-methyl-2-pyrrolidone and N, N-dimethylacetamide, in view of excellent stability and coatability of the electrode mixture.

The amount of the liquid in the electrode mixture may be determined in consideration of the coating property on the current collector, the film forming property after drying, and the like. In general, the ratio of the composition or binder to the above liquid is preferably 0.5 in terms of mass ratio: 99.5 to 20:80.

In order to be able to dissolve or disperse rapidly in the liquid, the composition or the binder is preferably used in a small particle size having an average particle size of 1000 μm or less, particularly 50 μm to 350 μm.

In order to further improve the adhesion to the current collector, the electrode mixture may further contain an acrylic resin such as polymethacrylate or polymethyl methacrylate, a polyimide, a polyamide, a polyamideimide resin, or the like. Further, a crosslinking agent may be added, and a crosslinked structure may be formed by irradiation with radiation such as gamma rays or electron rays. The crosslinking treatment method is not limited to irradiation of radiation, and may be other crosslinking methods, for example, a method in which an amine group-containing compound, a cyanurate group-containing compound, or the like capable of thermal crosslinking is added to cause thermal crosslinking.

In order to improve the dispersion stability of the electrode paste, a dispersing agent such as a resin-based or cationic surfactant or a nonionic surfactant having a surface active effect may be added to the electrode mixture.

The content of the composition or binder in the electrode mixture is preferably 0.1 to 20 mass%, more preferably 1 to 10 mass% based on the mass of the electrode mixture.

As a method for preparing the electrode mixture, there is a method of dispersing and mixing the powder electrode material in a solution or dispersion obtained by dissolving or dispersing the composition or binder in the above liquid. Then, the obtained electrode mixture is uniformly applied to a current collector such as a metal foil or a metal mesh, and dried, and if necessary, pressed to form a thin electrode material layer on the current collector, thereby forming a thin film-like electrode.

Alternatively, the electrode mixture may be prepared by mixing the powder of the composition or binder with the powder of the electrode material, and then adding the liquid. Alternatively, the electrode sheet may be produced by heating and melting a powder of the composition or binder and a powder of the electrode material, extruding the mixture by an extruder, preparing an electrode mixture of a film in advance, and bonding the electrode mixture to a current collector coated with a conductive binder or a general-purpose organic solvent. In addition, a solution or dispersion of the powder of the composition or binder and the powder of the electrode material may be applied to the electrode material preformed in advance. Thus, the method of application as the composition or binder is not particularly limited.

The electrode of the present invention contains the above composition or binder. Since the electrode of the present invention contains the composition or the binder, even if the powder electrode material is thick-coated, wound and pressed for the purpose of increasing the density, the electrode is not broken, and the powder electrode material is not peeled off and peeled off from the current collector. In addition, the electrode of the present invention is also excellent in resistance to swelling with an electrolyte. Further, by using the above electrode, it is possible to manufacture a secondary battery which is difficult to increase the resistance value even when stored at a high temperature and which can maintain a sufficient discharge capacity even when charge and discharge are repeated.

The electrode preferably includes a current collector and an electrode material layer formed on the current collector and containing the powder electrode material and the composition or binder. The electrode may be a positive electrode or a negative electrode, and is preferably a positive electrode.

Examples of the current collectors (positive electrode current collector and negative electrode current collector) include metal foils or metal meshes of iron, stainless steel, copper, aluminum, nickel, titanium, and the like. Among them, aluminum foil or the like is preferable as the positive electrode current collector, and copper foil or the like is preferable as the negative electrode current collector.

The electrode of the present invention can be manufactured by the above-described method, for example. Since the electrode mixture is excellent in coatability, an electrode having a smooth and uniform-thickness electrode material layer can be easily produced by using the electrode mixture to produce the electrode of the present invention.

The secondary battery of the present invention includes the above-described electrode. In the secondary battery of the present invention, at least one of the positive electrode and the negative electrode may be the electrode, and the positive electrode is preferably the electrode. The secondary battery is preferably a lithium ion secondary battery. The secondary battery of the present invention exhibits a low resistance increase rate and a high capacity retention rate.

The secondary battery of the present invention preferably further comprises a nonaqueous electrolyte. The nonaqueous electrolyte is not particularly limited, and known hydrocarbon solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ -butyrolactone, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, dimethyl carbonate, diethyl carbonate, and methylethyl carbonate can be used; 1 or 2 or more of fluorosolvents such as fluoroethylene carbonate, fluoroether and fluorinated carbonate. The electrolyte may be any of conventionally known electrolytes, and LiClO₄、LiAsF₆、LiPF₆、LiBF₄、LiCl、LiBr、CH₃SO₃Li、CF₃SO₃Li、 cesium carbonate or the like may be used.

A separator may be interposed between the positive electrode and the negative electrode. As the separator, a conventionally known separator may be used, or a separator in which the above composition or binder is used for coating may be used.

It is also preferable to use the above composition or binder in at least 1 of the positive electrode, the negative electrode, and the separator of a secondary battery (preferably a lithium ion secondary battery).

A film for a secondary battery comprising the above composition or binder is also one of preferred embodiments of the present invention.

A laminate for a secondary battery comprising a substrate and a layer formed on the substrate and containing the composition or binder is also one of the preferred embodiments of the present invention. Examples of the substrate include materials exemplified as the current collector, and known substrates (porous films and the like) used for separators of secondary batteries.

While the embodiments have been described above, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the claims.

Examples

Next, embodiments of the present invention will be described with reference to examples, but the present invention is not limited to the examples.

The values of the examples were measured by the following methods.

(Polymer composition)

The content of the pentenoic acid unit in PVdF can be measured by esterifying the carboxyl group of the pentenoic acid unit to convert it into an ester group and then analyzing the esterified PVdF by ¹ H-NMR. Specifically, 400mg of PVdF, 10mg of trimethylsilyl diazomethane and 3mg of methanol were reacted at 25℃for 12 hours, the resulting polymer was washed with methanol, dried at 70℃for 24 hours in vacuo, and the dried polymer was analyzed by ¹ H-NMR and found from the spectrum of 3.7ppm by ¹ H-NMR.

The content of acrylic acid units in PVdF was determined by acid-base titration of carboxyl groups. Specifically, about 0.5g of PVdF is dissolved in acetone at a temperature of 70℃to 80 ℃. To avoid solidification of PVdF, 5ml of water was added dropwise with vigorous stirring. Titration was performed with an aqueous NaOH solution having a concentration of 0.1N at a neutral shift at about-270 mV until complete neutralization of acidity. The content of acrylic acid units was calculated by determining the content of acrylic acid units contained in 1g of PVdF from the measurement result.

The ratio of VdF to TFE of the VdF/TFE copolymer was determined by ¹⁹ F-NMR measurement of DMF-d ₇ solution of the polymer using an NMR analyzer (manufactured by Agilent Technologies Co., ltd., VNS400 MHz).

The area (A, B, C, D) of the following peak was determined by ¹⁹ F-NMR measurement, and the ratio of VdF to TFE was calculated.

A: area of peak of-86 ppm to-98 ppm

B: area of peak of-105 ppm to-118 ppm

C: area of peak of-119 ppm to-122 ppm

D: area of peak of-122 ppm to-126 ppm

Proportion of VdF: XVdF = (4a+2b)/(4a+3b+2c+2d) ×100[ mol% ]

Ratio of TFE: x _TFE = (b+2c+2d)/(4a+3b+2c+2d) ×100[ mol% ]

The content of the CTFE unit and the HFP unit in the PVdF or VdF polymer was determined by a method such as quantification of chlorine content, ¹⁹ F-NMR measurement using an NMR analyzer (manufactured by Agilent Technologies Co., ltd., VNS400 MHz), or the like.

(Solution viscosity)

An NMP solution (5 mass%) of PVdF or VdF polymer (binder) was prepared. The viscosity of the NMP solution after 10 minutes from the start of the measurement was measured using a type B viscometer (TV-10M, manufactured by Dong machine industries Co., ltd.) at 25℃with a rotor No. M4 at a rotation speed of 6 rpm.

(Weight average molecular weight)

The measurement was performed by Gel Permeation Chromatography (GPC). Using AS-8010, CO-8020, a column (3 GMHHR-H were connected in series) and RID-10A, which was manufactured by Shimadzu corporation, dimethylformamide (DMF) was introduced AS a solvent at a flow rate of 1.0 ml/min, and the results were calculated from the measured data (reference: polystyrene).

(Melting point)

Using a Differential Scanning Calorimetric (DSC) apparatus, the temperature was raised from 30 ℃ to 220 ℃ at a rate of 10 ℃/min, then lowered to 30 ℃ at a rate of 10 ℃/min, and again raised to 220 ℃ at a rate of 10 ℃/min, and the temperature relative to the maximum in the heat of fusion curve at that time was determined as the melting point.

(Electrolyte swelling resistance)

A NMP solution (8 mass%) of the composition (binder) was poured onto a glass petri dish, and vacuum-dried at 100℃for 6 hours to prepare a film having a thickness of 200. Mu.m. The obtained film was cut to a size of 10mm Φ, placed in a sample bottle filled with an electrolyte (a solution obtained by dissolving LiPF ₆ in a solvent of 3/7 (volume ratio) of ethylene carbonate to ethylmethyl carbonate at a concentration of 1M), and after standing at 60 ℃ for 1 week, the weight increase rate was determined by the following formula, whereby the electrolyte swelling resistance was evaluated.

Weight increase ratio (%) = (film weight after electrolyte impregnation/film weight before electrolyte impregnation) ×100

(Viscosity Change Rate of Positive electrode mixture)

The viscosity of the positive electrode mixture after 10 minutes from the start of measurement was measured using a type B viscometer (TV-10M, manufactured by Dong machine industries Co., ltd.) at 25℃with a rotor No. M4 at a rotation speed of 6 rpm. The viscosity change rate (Xn) was determined from the viscosity (η0) of the positive electrode mixture measured immediately after the preparation of the positive electrode mixture and the viscosity (ηn) after 24 hours after the preparation of the mixture, according to the following formula.

Xn＝ηn/η0×100[％]

(Density of the Positive electrode Material layer)

The area, film thickness and weight of the positive electrode material layer were measured with respect to the density of the positive electrode material layer, and calculated from these values.

(Peel strength of the positive electrode material layer of the positive electrode and the positive electrode collector)

The positive electrode was cut, whereby a test piece of 1.2 cm. Times.7.0 cm was produced. After fixing the positive electrode material layer side of the test piece to a movable jig with a double-sided tape, the tape was stuck to the surface of the positive electrode current collector, and the tape was stretched at a speed of 100 mm/min at 90 degrees, and the stress (N/cm) at this time was measured by an Autograph. The load cell of the Autograph uses 1N.

< Softness of Positive electrode >

The positive electrode was cut to prepare a test piece of 2cm×10cm, and wound around a round bar of 2.0mm in diameter, and the positive electrode was visually confirmed and evaluated according to the following criteria.

And (2) the following steps: no cracking and breakage were observed.

Delta: cracks were observed in the positive electrode material layer, but no fracture of the positive electrode material layer and the current collector was observed.

X: the positive electrode material layer and the current collector are broken.

The following PVdF (A) and VdF polymer (B) were used in examples and comparative examples.

PVdF(A1)

In an autoclave having an internal volume of 2.5 liters, vdF668g was charged together with 1546g of pure water, 1.5g of methyl cellulose, 1ml of 4-pentenoic acid, 2ml of methanol and 1g of di-n-propyl peroxydicarbonate, and after heating to 31℃for 1.5 hours, the temperature was maintained at 31℃for 9 hours. The highest reached pressure during this period was 7MPaG.

The polymerization was ended 9 hours after the completion of the temperature rise to 31 ℃. After completion of the polymerization, the obtained polymer slurry was recovered, dehydrated and washed with water, and further dried at 118℃for 12 hours to obtain PVdF powder (PVdF (A1)).

Physical properties of PVdF (A1) are shown.

PVdF containing 4-pentenoic acid units

Content of 4-pentenoic acid units: 0.09 mol%

Solution viscosity: 712 mPa.s

Weight average molecular weight: 1160000

Melting point: 173 DEG C

PVdF(A2)

In an autoclave having an internal volume of 2 liters, vdF357g was charged together with 700g of pure water, 0.35g of methyl cellulose, 0.8ml of 4-pentenoic acid, 1.0ml of methanol and 2.0g of t-butyl peroxy-2-ethylhexanoate, and after heating to 72℃for 1.5 hours, the temperature was maintained at 72℃for 12 hours. The highest reached pressure during this period was 7.9MPaG.

The polymerization was ended 18 hours after the completion of the temperature rise to 72 ℃. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated and washed with water, and further dried at 118℃for 12 hours to obtain a PVdF powder.

Physical properties of PVdF (A2) are shown.

PVdF containing 4-pentenoic acid units

Content of 4-pentenoic acid units: 0.21 mol%

Solution viscosity: 321 mPas

Weight average molecular weight: 700000

Melting point: 161 DEG C

PVdF(A3)

PVdF containing acrylic acid unit

Content of acrylic acid unit: 1.0 mol%

Solution viscosity: 644 mPa.s

Weight average molecular weight: 1000000

Melting point: 164 DEG C

VdF Polymer (B1)

To a 4 liter autoclave, 1340g of pure water, 0.75g of methylcellulose, 1280g of perfluorocyclobutane (C318), 218g of a mixed gas having a VdF/TFE molar ratio of 94.3/5.7 (mol%) were added, the temperature was adjusted to 37℃and then 0.699g of di-sec-butyl peroxydicarbonate, 6g of methanol and 0.88g of ethyl acetate were added, 384g of a mixed gas having a VdF/TFE molar ratio of 85/15 (mol%) was added so that the cell pressure became 1.4MPaG, and after 11 hours from the start of the reaction, the inside of the cell was depressurized to obtain a VdF/TFE copolymer.

The physical properties of the obtained VdF/TFE copolymer (VdF polymer (B1)) are shown.

VdF/TFE copolymer

VdF/tfe=83.3/16.7 (mol%)

Solution viscosity: 847 mPas

Weight average molecular weight: 1160000

Melting point: 131 DEG C

VdF Polymer (B2)

To an autoclave having an internal volume of 4 liters, 1340g of pure water, 0.67g of methylcellulose, 318 1280g of C, 147g of a mixed gas having a vdF/TFE molar ratio of 70/30 (mol%), after the temperature was adjusted to 37 ℃, 1.368g of di-sec-butyl peroxydicarbonate, 8g of methanol, and 2g of ethyl acetate were added, 384g of a mixed gas having a vdF/TFE molar ratio of 60/40 (mol%) was added so that the tank pressure became 1MPaG, and after the reaction was started, the pressure in the tank was released for 4 hours to obtain a vdF/TFE copolymer.

The physical properties of the obtained VdF/TFE copolymer (VdF polymer (B2)) are shown.

VdF/TFE copolymer

VdF/tfe=60.0/40.0 (mol%)

Solution viscosity: 1548 mPas

Weight average molecular weight: 860000

Melting point: 164 DEG C

VdF Polymer (B3)

PVdF containing CTFE unit

Content of CTFE units: 2.0 mol%

Solution viscosity: 433 mPas

Weight average molecular weight: 850000

Melting point: 168 DEG C

VdF Polymer (B4)

PVdF containing HFP unit

Content of HFP units: 5.0 mol%

Solution viscosity: 269 mPa.s

Weight average molecular weight: 730000

Melting point: 133 DEG C

VdF Polymer (B5)

PVdF (VdF homopolymer)

Solution viscosity: 2400 mPas

Weight average molecular weight: 1800000

Melting point: 171 DEG C

Examples 1 to 10 and comparative example 1

(Preparation of the composition)

The composition (binder solution) containing PVdF (a) and VdF polymer (B) was prepared by dissolving PVdF (a) and VdF polymer (B) in N-methyl-2-pyrrolidone (NMP) so that the concentration of the binder (PVdF (a) and VdF polymer (B)) in the NMP solution was 8 mass% and the mass ratio of PVdF (a) to VdF polymer (B) was as shown in table 1.

(Preparation of Positive electrode mixture)

The positive electrode active material (NMC (811) (LiNi _0.8Mn_0.1Co_0.1O₂)) and the conductive agent (acetylene black (AB)) were added to the above-obtained composition, and the mixture was thoroughly mixed with a stirrer to prepare a positive electrode mixture. The mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode mixture was 96/2/2. The solid content concentration in the positive electrode mixture was 70 mass%.

(Preparation of positive electrode)

The positive electrode material layer and the positive electrode collector were prepared by uniformly applying the obtained positive electrode mixture to one surface of a positive electrode current collector (aluminum foil having a thickness of 20 μm) so that the applied amount was 22.5mg/cm ², completely volatilizing NMP, and then pressing the positive electrode mixture by applying a pressure of 10t using a roll press. The density of the positive electrode material layer was 2.75g/cc.

The evaluation results are shown in Table 1.

TABLE 1

Claims

1. A composition comprising polyvinylidene fluoride (A) and a vinylidene fluoride polymer (B), wherein the vinylidene fluoride polymer (B) does not include polyvinylidene fluoride (A),

The polyvinylidene fluoride (a) contains a vinylidene fluoride unit and has the formula (1): a pentenoic acid unit represented by CH ₂=CH-(CH₂)₂ -COOY, wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations, the content of the vinylidene fluoride unit of the polyvinylidene fluoride (A) is 98.5 mol% to 99.99 mol% relative to the entire monomer units of the polyvinylidene fluoride (A), the content of the pentenoic acid unit of the polyvinylidene fluoride (A) is 0.01 mol% to 1.5 mol% relative to the entire monomer units of the polyvinylidene fluoride (A),

The vinylidene fluoride polymer (B) contains vinylidene fluoride units and fluorinated monomer units,

The fluorinated monomer unit is a unit based on at least one monomer selected from the group consisting of tetrafluoroethylene and hexafluoropropylene,

The mass ratio (A)/(B) of the polyvinylidene fluoride (A) to the vinylidene fluoride polymer (B) is 95/5 to 45/55.

2. The composition according to claim 1, wherein the content of vinylidene fluoride units of the vinylidene fluoride polymer (B) is 57.0 mol% to 99.9 mol% relative to the total monomer units of the vinylidene fluoride polymer (B),

The content of the fluorinated monomer unit of the vinylidene fluoride polymer (B) is 0.1 mol% to 43.0 mol% with respect to the total monomer units of the vinylidene fluoride polymer (B).

3. A binder comprising the composition of claim 1 or 2.

4. An electrode mix comprising the composition of claim 1 or 2 or the binder of claim 3, a powder electrode material, and an aqueous or nonaqueous solvent.

5. An electrode comprising the composition of claim 1 or 2 or the binder of claim 3.

6. A secondary battery comprising the electrode according to claim 5.